L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8”. Use chart below to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.

I just ran the chain over the biggest sprocket , over the chain ring through the mech and pulled the ends together until I thought the mech was stretched to it's limit, backed it off to give one link breathing space and that seems to have worked fine.